The printing industry in general finds may applications for the use of water-based inks and overprint varnishes as a means of meeting increasingly stringent solvent effluent regulations. Present day water-based inks often fail to satisfy these regulations as well as the necessary printability, stability, and performance properties required for commercial inks. For example, the various ethyleneacrylic acid copolymer salts of U.S. Pat. No. 3,607,813, the disclosure of which is incorporated herein by reference (for the printing process descriptions therein), in addition to requiring complex polymer and ink preparations, lack in performance on certain substrates. Other such aqueous or semiaqueous systems proposed for printing inks contain polymers such as styrene-butadiene or polyacrylate latex systems but these systems also have serious drawbacks including being non-water dispersible after short drying periods which complicates equipment clean up. Other water soluble or dispersible polymers suggested for printing ink use are discussed in U.S. Pat. No. 4,072,644.
In the commercial use of polymeric coatings such as paints, inks, and the like that are applied from water solutions or dispersion, it is an economic advantage to have a material that is initially resoluble or redispersable in water even after it has dried to a film. This allows facile water clean-up of coating and printing equipment and other items such as mixing vessels and handling equipments such as sampling cups and viscosity measurement instruments. However, it is also an advantage if the resulting coating can be converted in a short time to a highly water-resistant surface on the coated or printed product. These seemingly contradictory requirements are not well met by current commercial materials and post treatment techniques. The best results published to date invovle pH change, i.e., evaporation of ammonia or similar base, to yield insoluble coatings, however, the pH adjustments can be a problem with a highly volatile base and odorous emissions and equipment corrosion often result.
It has now been discovered that far superior ink systems having these desirable properties can be prepared through the use of certain water-insoluble metal chelates which are sequestered at a pH of from about 6.0 to about 8.0 in water dispersions of the water-dispersible polymers, and which then behave as stable, "sequestered" hydrophobes as long as water is present. Evaporation of the water and subsequent holding and/or heating, usually as a part of the production drying, gives a highly water-resistant film which is essentialy otherwise unchanged.
It is noted that the application of heat to a film of the ink, particularly to a film containing some water such as a freshly printed film, will enhance the insolubilizing action of the metal complexed acetylacetonates which enhancement is manifest in greater insolubilization. The heating is preferably at temperatures of from about 50.degree. C. to about 100.degree. C., for about 2.0 to about 40 seconds, and most preferably of from about 70.degree. C. to about 90.degree. C. for about 4.0 to about 10.0 seconds.
It is known that free multivalent cations will cause such water-resistance to develop in polymer films post-treated in separate, extrinsic operations with solutions containing multivalent cations (see defensive publication T-949001); however, stable, water-insoluble metal chelates would not be expected to be sequesterable in such a system to provide a highly convenient mechanism for insolubilization, much less give equivalent or superior water resistance upon drying.
The present invention, in addition to providing a unique ink system having an early redispersable phase which can be rendered essentially nondispersable, also provides marked improvements in the preparation, stability, and performance of water-based inks for printing and coating, particularly in regard to flow-out, color development and pick-up on the printing rollers and other machine elements.